On Fri, Aug 13, 2021 at 11:55:19PM +0300, Nir Soffer wrote: > Both qemu and qemu-img use writeback cache mode by default, which is > already documented in qemu(1). qemu-nbd uses writethrough cache mode by > default, and the default cache mode is not documented. > > According to the qemu-nbd(8): > > --cache=CACHE > The cache mode to be used with the file. See the > documentation of the emulator's -drive cache=... option for > allowed values. > > qemu(1) says: > > The default mode is cache=writeback. > > So users have no reason to assume that qemu-nbd is using writethough > cache mode. The only hint is the painfully slow writing when using the > defaults.
Oh, good catch. Unfortunately too late for 6.1 proper, but I'll add qemu-stable in cc and queue this through my NBD tree for 6.2. > Users can avoid the issue by using --cache=writeback[1] but the defaults > should give good performance for the common use case. > > [1] https://bugzilla.redhat.com/1990656 > > Signed-off-by: Nir Soffer <nsof...@redhat.com> > --- Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
